Electrical schoolwork...

Stephen Kormilo stephen at kormilo.ca
Tue Feb 10 11:31:31 EST 2004

At 01:57 AM 10/02/04 -0500, quattro-request at audifans.com wrote:

>Date: Tue, 10 Feb 2004 01:57:48 -0500
>From: Huw Powell <audi at humanspeakers.com>
>Subject: Re: Electrical schoolwork...
>To: SJ <syljay at optonline.net>
>Cc: quattro at audifans.com
>Message-ID: <402880EC.1090505 at humanspeakers.com>
>Content-Type: text/plain; charset=us-ascii; format=flowed
>I prefer this answer to Ti's current based calculation.

Well, notwithstanding your preference, Ti's answer is correct.  A 
resistance of ~700 k-ohms will result in a _very_ dim LED ;-)

The brightness of the LED depends on the _current_ through it, and, just 
like other types of diodes, the voltage across a forward-biased LED remains 
nearly constant, typically at around 2.5V at normal operating currents, 
typically ~20 mA.  So a resistance in the 500 ohm range will do the 
job.  Exact resistance values are far from being critical, unless they 
result in currents that exceed the maximum LED current rating, typically 
~50 mA or so,

The LED resistance figure given in the original poster's query is totally 
meaningless and was probably measured with an ohmmeter.

>I would recommend testing your choice "out in the open" before
>rebuilding a fistful of switches, etc., to make sure you got it right.

I would recommend this approach to determine the desired brightness 
level.  Use a fixed value resistor of ~330 ohms in series with a ~500 ohm 
variable resistor to 'play' with the brightness level.  Don't forget that 
LEDs are polarity sensitive.  They will not light when 
reverse-biased.  Connect the LED Anode to +'ve through the resistor and the 
LED Cathode to Ground.

>SJ wrote:
> >>From: "Louis-Alain RICHARD" <laraa at sympatico.ca>
> >>
> >>Resistance value of the LEDs: 150 000 ohms
> >>Ideal voltage at the LEDs: between 2.5V and 3V.
> >>Electrical voltage at the switches: 13.6V, permanent, switched by the
> >
> >
> > ***** For 3 volts
> >  x/150k = 12v/3v
> > x = (12v/3v)*150K
> > x = 600K
> >
> >
> > for 2.5 volts
> > x/150k = 12v/2.5v
> > x = (12v/2.5v)*150k
> > x = 720K
> >
> > You dont need an exact value. Anything close will do.
>Huw Powell

Former (now retired) Electronics Technology Instructor.

Stephen Kormilo
Winnipeg, MB

Silver 2002 Audi A4 3.0QM
Now departed:   Silver '98.5 Audi A4 2.8QM
                            Black '85 Mazda Rx7 GSL-SE

email: skormilo at rrc.mb.ca
           stephen at kormilo.ca

More information about the quattro mailing list